This invention relates to an internal reforming type fuel cell. More particularly, it relates to an internal reforming type fuel cell which is supplied with a fuel gas reformed by a catalyst and an oxidizing gas, to cause electrochemical reactions.
FIG. 1 shows an internal-reforming molten-carbonate type fuel cell (hereinbelow, simply termed `fuel cell`) in a prior art. In the figure, numeral 1 designates an electrolyte matrix, numeral 2 a fuel gas side electrode, and numeral 3 an oxidizing gas side electrode. Numeral 4 indicates a fuel gas side perforated plate which supports the fuel gas side electrode 2 and which separates a reforming catalyst to be described later, from the fuel gas side electrode 2. Numeral 5 indicates an oxidizing gas side perforated plate which supports the oxidizing gas side electrode 3. Shown at numeral 6 is a separator plate, which separates and defines a fuel gas side gas passage and an oxidizing gas side gas passage. Further, in stacking a plurality of unit fuel cells 7 each of which is constructed of the electrolyte matrix 1, the fuel gas side electrode 2, the oxidizing gas side electrode 3, the fuel gas side perforated plate 4 and the oxidizing gas side perforated plate 5, and the separated plates 6 function to connect the unit fuel cells 7 in electrical series. The fuel gas side gas passage 8 and the oxidizing gas side gas passage 9 are defined by the separator plates 6. The reforming catalyst is shown at numeral 10, and it promotes the reaction of decomposing hydrocarbon or alcohols to produce hydrogen.
With the above construction, a fuel gas whose principal ingredients are hydrocarbon or alcohols and steam and also an oxidizing gas whose principal ingredients are oxygen and carbon dioxide are supplied to the fuel cell in cruciform flow fashion, and they are respectively introduced into the fuel gas side gas passage 8 and the oxidizing gas side gas passage 9. The hydrocarbon or alcohols in the fuel gas fed into the fuel gas side gas passage 8 is/are modified into a fuel gas whose principal ingredients are hydrogen and carbon monoxide, by the action of the reforming catalyst 10 contained in the fuel gas side gas passage 8 and owing to chemical reactions as indicated by the following formulas. The reactions form an endothermic reaction as a whole, and thermal energy accessorily produced by the fuel cell is directly utilized. EQU Hydrocarbon +H.sub.2 O.fwdarw.H.sub.2, CO, CO.sub.2, CH.sub.4 ( 1) EQU Alcohols +H.sub.2 O.fwdarw.H.sub.2, CO, CO.sub.2, CH.sub.4 ( 2) EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 ( 3) EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 ( 4)
Hydrogen produced within the fuel gas side gas passage 8, and oxygen and carbon dioxide in the oxidizing gas diffuse through the pores of the fuel gas side perforated plate 4 and the oxidizing gas side perforated plate 5 respectively. They cause electrochemical reactions as indicated by the following formulas in the fuel gas side electrode 2 and the oxidizing gas side electrode 3 respectively:
(fuel gas side electrode) EQU H.sub.2 +CO.sub.3.sup.2- .fwdarw.H.sub.2 O+CO.sub.2 +2e (5) PA1 (Oxidizing gas side electrode) EQU 1/2O.sub.2 +CO.sub.2 +2e.fwdarw.CO.sub.3.sup.2- ( 6)
Through these series of chemical and electrochemical reactions, chemical energy possessed by the fuel gas is converted into electric energy and the thermal energy is produced accessorily.
In this manner, in the internal reforming type fuel cell, waste heat accessorily produced by the electrochemical reactions based on the formulas (5) and (6) is directly utilized as reaction heat necessary for the reforming reactions, and the cooling of the fuel cell is conjointly performed, thereby to bring forth the merit that the efficient utilization of the waste heat is made possible as a system. Besides, a heat exchanger may be small in size owing to the mitigation of the cooling load of the fuel cell, and a fuel processor for decomposing hydrocarbon or alcohols need not be disposed outside, so that the miniaturization of the system becomes possible advantageously.
Further, the reforming reactions of Formulas (1)-(4) and the electrochemical reaction of Formula (5) are carried out in parallel in the same space, whereby hydrogen produced by the reforming reactions (1)-(4) is continuously consumed by the electrochemcial reaction of Formula (5), and steam required for causing the reforming reactions of Formulas (1)-(4) to proceed is continuously supplied from the electrochemical reaction of Formula (5). By carrying out both the reactions in parallel in this manner, there are attained the merits that the proceeding of the reforming reactions of Formulas (1)-(4) is promoted more than in a case of performing them alone, and that the quantity of undecomposed hydrocarbon or alcohols at the outlet of a reforming region can be reduced.
Here, the reforming catalyst 10 is, for example, an active material such as nickel supported on a carrier whose principal ingredient is alumina, magnesia or the like. In order to maintain the activity of the reforming catalyst 10, the fuel cell needs to be operated while the oxidation of the active material such as nickel as indicated by the following formula is prevented: EQU Ni+H.sub.2 O.fwdarw.NiO+H.sub.2 ( 7)
The decomposition of hydrocarbon or alcohols and the production of hydrogen, which employ a conventional reforming catalyst, are carried out by adding steam as indicated by Formulas (1)-(4). Since, in this case, the oxidation of the active material such as nickel is prevented by the produced hydrogen, an operation stable for a long term is possible.
In the internal-reforming molten-carbonate type fuel cell, however, hydrogen produced by Formulas (1)-(4) is consumed in parallel with the production, to produce steam as indicated by Formula (5). Accordingly, the concentration of hydrogen lowers, and the oxidation of the active material of the reforming catalyst 10, e.g., nickel takes place, so that the activity of the reforming catalyst 10 is prone to lower.
The conditions of such oxidation of the active material differ depending upon the kind of the active material, the kind of the carrier, temperature, etc. Regarding the catalyst which employs nickel as the active material, it is known as one criterion that when the ratio of the concentration of water vapor to the concentration of hydrogen exceeds 10-20, the oxidation of the nickel takes place to lower the activity of the catalyst.
The ratio of the water vapor concentration to the hydrogen concentration has a distribution in the flowing direction of the fuel gas. Usually, the ratio of the water vapor concentration to the hydrogen concentration becomes the greatest in the inlet portion of the fuel gas, and the ratio in that portion is the most relevant to the stability of the reforming catalyst 10. There are two methods for holding this ratio small. One of them is to make active the reactions of decomposing hydrocarbon and producing hydrogen in Formulas (1)-(4), that is, to raise the activity of the reforming catalyst 10 and to increase the packing quantity thereof. The other is to suppress the electrochemical reaction of consuming hydrogen in Formula (5), that is, to output only a small current.
In the prior-art fuel cell of the internal-reforming molten-carbonate type, the above requirement has been met in such a way that the packing quantity of the reforming catalyst 10 is made several times larger than in an ordinary reforming reactor.
WIth the prior-art internal reforming type fuel cell thus far described, packing a large quantity of reforming catalyst therein has been necessary for providing a high output stably over a long term. As a result, the dimensions of the fuel gas side gas passage have been enlarged, presenting the problem that the internal reforming type fuel cell becomes large in size as a whole.